The invention relates to a fuel cell system having a fuel cell and a catalytic burner connected downstream therefrom, and a method for operating such a fuel cell system.
A fuel cell system and method of this type, with a downstream catalytic burner, are known, for example, from German patent document DE 197 55 116, in which the fuel cell is initially fed with a fuel-containing medium and an oxygen-containing medium. The fuel-containing medium used is generally a liquid or gaseous hydrocarbon compound, preferably a low-molecular-weight hydrocarbon compound, such as for example methanol, which can be obtained by reforming higher molecular weight compounds. The oxygen-containing medium used may, for example, be air.
After they have passed through the fuel cell and are discharged therefrom, the media are fed to a catalytic burner for combustion. The heat obtained during the combustion can be utilized in the fuel cell system or can be discharged for use in some other way.
One problem in such systems is that, during the start-up phase of each operating cycle of the fuel cell system, the catalyst of the catalytic burner is still cold, and exhibits a low activity. Consequently combustion is either insufficient, or does not take place at all, resulting in increased pollutant emissions and delayed response of the burner, associated with delayed emission of heat.
U.S. Pat. No. 5,686,196 discloses a hydrogen store for storing hydrogen which is generated during an operating cycle of the fuel cell system. This hydrogen is fed to the fuel-containing medium for de-sulphurization in particular during the start-up and stopping phase.
It is an object of the present invention to improve the start-up performance of a fuel cell system, particularly its catalytic burner, and to reduce its emissions.
This and other objects and advantages are achieved by the fuel cell system according to the invention, which includes a feed device for feeding an additional hydrogen-containing medium (which is more highly reactive than the media discharged from the fuel cell) to the catalytic burner at the start of an operating cycle of the fuel cell system. It should be noted in this regard that a catalytic burner is usually started up by means of hydrocarbons. As a result of the inventive feed of hydrogen, it is possible to pre-heat the catalyst in the catalytic burner to such an extent that these hydrocarbons can be reacted without any emissions. As a result, catalytic combustion is initiated even at the relatively low temperatures which prevail in the catalytic burner during the start-up phase, and which generally correspond to ambient temperature. Furthermore, during the combustion of the additional hydrogen-containing medium, there are far fewer undesirable emissions in the (still) cold catalytic burner than would be the case if hydrocarbons were being burnt. Therefore, a considerable contribution is made to environmental protection.
The reaction constituent used for the additional hydrogen-containing medium is an oxygen-containing medium, preferably air, which is fed in the usual way via the fuel cell. Alternatively, the fuel cell system may also have a separate feed device for feeding the oxygen-containing medium to the catalytic burner.
To achieve a favorable overall energy balance of the fuel cell system, the additional hydrogen-containing medium is supplied only in the start-up phase.
After combustion of the additional hydrogen-containing medium has provided sufficient heating of the catalytic burner, further heating can take place by the addition of a fuel-containing medium, such as for example methanol; therefore it is ultimately possible to switch to normal operation. In this phase, only a relatively small quantity of the additional hydrogen-containing medium (or none at all) is still required. Therefore, it is advantageous if the fuel cell system has a control unit which controls the feed device for the additional hydrogen-containing medium. In this way, the supplied quantity of additional hydrogen-containing medium can be adapted to prevailing conditions, and if appropriate can be stopped altogether. In this context, it should be noted that in normal operation the exhaust gas from the fuel cell anode (residual hydrogen) is reacted. If appropriate, it is possible to feed additional hydrocarbon, for example methanol, to the catalytic burner.
In bringing the catalytic burner to a desired temperature, the control device regulates the feed device (and therefore the feed of additional hydrogen-containing medium) as a function of the temperature of the catalytic burner. For this purpose, a temperature sensor may be arranged on the latter. If an arrangement of this type is not expedient, it is also possible to measure the temperature at other points in the fuel cell system where the temperature correlates with that of the catalytic burner. Finally, the control may also take place at low cost purely as a function of time, if the time required for the catalytic burner to reach the desired temperature is known with sufficient accuracy.
In order always to have a sufficient hydrogen-containing medium available, the fuel cell system has a store for additional hydrogen-containing medium, which is in communication with the feed device. The additional hydrogen-containing medium contains, for example, reformate, which may constitute a substantial portion thereof. Alternatively, or in addition, it may contain or consist of hydrogen obtained in a preceding operating cycle of the fuel cell system.
The additional hydrogen-containing medium can be passed from the feed device directly to the catalytic burner. However, it may also be advantageous to arrange a liquid separator in the feed device or between the feed device and the catalytic burner. As a result, if appropriate, condensate can be separated out. If the liquid separator is arranged between feed device and the catalytic burner, it is also possible to separate out condensate contained in the oxygen-containing medium which is supplied for combustion.
As a further start-up aid, a hot point may be arranged in the catalytic burner, locally or over a large area, in order to assure reliable starting of the reaction with hydrogen, even at low temperatures. For this purpose, a means for increasing the temperature of the catalytic burner locally or over a large area is situated on or in the catalytic burner. Alternatively, the additional hydrogen-containing medium or the oxygen-containing medium may also be heated.
The means for increasing the temperature is advantageously designed as an electrical heater means, e.g. as a spark plug.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.